Tidjane Thiam, the chief executive of Credit Suisse, has asked the company’s board to reduce his bonus, days after the Swiss bank reported a multibillion-dollar loss in the fourth quarter. “I have asked the board of directors for a significant reduction in my bonus,” Mr. Thiam said in a statement issued by the bank on Sunday. Mr. Thiam, who joined the bank in July, did not indicate the size of the reduction in his bonus, but said his was the largest reduction within the management team.